Automatic gain control



April 13, 1937. c. J. FRANKS AUTOMATIC (FAIN CONTROL 2 Sheets-Sheet 1Filed July 11, 1933 Fay -Z.

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Patented Apr. 13,1937 g l V 1 UNITED STATES PATENT QFFIE Y AUTOMATICGAIN CONTROL Christopher J. Franks, Boonton, N. 3., assignor, by mesneassignments, to Radio Corporation of America, New York, N. Y., acorporation of Delaware Application July 11, 1932,, Serial No. 679,947

' 2 Claims. (01. 250-20) This invention relates to automatic gain con-Fig. 4 is a fragmentary circuit diagramof a trol circuits of the type inwhich the carrier voltreceiver embodying the invention; and age isrectified to obtain a direct current poten- Fig. 5 is a family ofreceiver input-detector tial for varying the gain of an amplifier. inputcurves plotted from data obtained for a 5 The automatic volume controlsystems cusplurality of adjustments of the resistance which 5 tomarilyemployed to control the gain of a radio controlled that fraction of theavailable gain conamplifier place the rectifier of the control systemtrol voltage that passed to the amplifier stage after the amplifier, andthe rectified potential is succeeding the gain control system. fed backto the amplifier. Such systems neces- In the drawings, the referencecharacters AG sarily result in a substantial variation in theoutidentify a collector structure of any desired form 0 put level sincethe control depends upon a variafor receiving carrier wave signals andtransfertion in the magnitude of the amplifier output ing the same toamplifier RF that includes a pluvoltage which is impressed upon therectifier and rality of cascaded amplifier tubes 1, 2, and 3. The uponth d t t coupling circuits between successive tubes may An object of thepresent invention is to probe of any desired construction, and the ampli15 vide an automatic gain control which is capab1e fication maybe at thereceived carrier frequency of giving a constant output level for allradio ind/0 at a in e med ate requ ncy. put voltages above the criticalvalue which, at According to the invention, the .automatic gain maximumgain, will give the desired output level. t ol voltage obtained b ficaion of the An object is to provide methods of and circuit amplifiedradio voltage E1 developed in a circuit 20 arrangements for automaticgain control which of the intermediate tube 2 is app t y t arecharacterized by a control of the gain of a n or more of the precedingtubes l, as is e portion of the radio amplifier which succeeds known pratic but also to one or more of the that point in a multistage radioamplifier at succeeding tubes 3 0f the mum-Stage p fi which an amplifiedradio voltage is passed to As shown in Fig. 1, the rad o Voltage E1 at25 the gain control system. More specifically, an the outputcircuit ofthe tube 2 is impressed upon object is to provide a multi-stageamplifier in the succeeding amplifier tube and also upon a which theradio voltage at anintermediate point rectifier 4 of the automatic gaincontrol system in the amplifier is rectified to obtain again conwhich isindicated generically at C. The direct trol voltage, andthe gain controlvoltage is imcurrentlpotential developed across the output 1' 3o pressedupon one or more stages preceding and sistance 5 of the rectifier ispassed, through the subsequent to the intermediate or control point. filer n w rk f rm by r i an es 6, 6' and A further object of the inventionis to provide a, condensers I, to the leads 8 that run to the conradioreceiver of the automatic gain control type, rol. grids of the amplifiertubes l and in which the gain control unit includes a ra- Any desiredportion of the control voltage is 35 dio amplifier for impressing uponthe control ele-. a e y adjustable n 9 and d W, to ments a radio voltageof greater magnitude and, the control grid of one or more of theamplifier if desired, more sharply: restricted t th fr tubes 3 whichfollow the tube 2. The last tube 3- quency of the desired carrier wavesignal than y Serve y as an a p it m y be 40 is the radio output of theamplifier stage which both a radio amplifier and detector, for p worksinto the control unit and into the deteca plate C c t e ector. When thetube 3 func-. t r, y tions only as an amplifier, the first tube ll ofThese and other objects and advantages of the the audio freque y systemAF acts as the deinvention will be apparent from the following eeerspecification when taken with the accompanying The operation and v a sof the inven- 4.5 drawings in which: tion will be apparent from thegeneralized curves. Fig. 1 is a somewhat diagrammatic i w of a, of Fig.2. With automatic bias applied to tubes circuit embodying the invention;I, and not to the tube or tubes 3, the relation? Fig. 2 is a curve sheetshowing the general rebetween the output level, or voltage E0, and thelationship between receiverinput and audio outsignal voltage E, will besubstantially as indicated put level with known systems and with a conbycurve 0A. As the carrier wave voltage introl system embodying theinvention; creases, the radio voltage Er at tube 2 increases, Fig. 3 isa curve showing the relation between thereby increasing the bias on thecontrol grids the grid bias and the gain of oneof the conof tubes 1 anddecreasing the gain of the amplitrolled tubes; a h r, At that normalsignal strengthEnwhich, 55

with maximum gain would give the desired output voltage, the gain hasbeen substantially reduced by the automatic volume control. A bettercontrol, such as shown by the line ONB, may be obtained by the knownsystems for delaying the operation of the automatic gain control untilthe carrier voltage rises to the normal value En, but even with suchsystems, the branch NB of the control curve must rise with increasingsignal strength since an increased radio voltage E1 must obtain at theoutput of the last controlled tube to produce that increase in rectifiedradio voltage which is required to reduce the gain as the signalstrength increases.

By applying the gain control voltage to a tube 3 which follows theintermediate point, tube 2, at which the control system is connectedacross the amplifier, the variation in the amplified radio voltage E1may be ofiset by corresponding variations in the gain of the tubes 3,thus giving a branch NN of the control curve which may be substantiallyflat over an extended range of signal voltages.

To prevent the reduction in gain for signals below the normal value En,the initial bias on the tube or tubes 3 may be such that the gainincreases as the automatic gain control first comes into action withincreasing signal strength, and then decreases. As shown in 'Fig. 3, thegaincontrol grid bias curve D changes curvature within the operatingrange of bias voltages and the initial bias aa may be so chosen that, asthe gain control component of the bias voltage increases, the gain ofthe portion of the controlled r amplifier succeeding the control unit Cincreases to counteract the decreased gain of the tubes I, thus giving anet gain-curve for signals below En that approximates the branch ON, andthen decreases to effect the reduction in gain that cannot be producedby applying the control to tubes preceding the control unit C.

It will be apparent, however, that the invention is generally applicableto known types of gain control systems to modify the relation betweenthe output level and the signal input in any desired manner. Thecompensation for the variation of the voltage El may be either greateror less than that necessary to maintain a constant amplifier output E0.The control of the gain of stages succeeding the control point may alsobe applied to systems which include some provision for delaying theaction of the gain control until the received signals exceed the normalinput voltage En- As shown in Fig. 4, the radio amplifier RF, which mayinclude one or more controlled amplifier tubes and, if desired, one ormore amplifier tubes on which the gain is not controlled automatically,is coupled in any appropriate manner to the input circuit of the radioamplifier tube 3. The input circuit is shown as an inductance l2 whichis tuned by a condenser I3, the blocking condenser 14 being connectedbetween the lower terminal of the inductance and ground to permit theapplication of a gain control bias through the lead Ill. The tube 3works into a combined diode detector and audio amplifier tube I5 whichmay be of the known Type 55, and the interstage coupling may be atransformer [6 having tuned primary and secondary windings. The detailsof the circuits of tube [5 do not constitute an essential part of theinvention and therefore a description of the same is not believed to benecessary, it being sufiicient to note that an audio frequency voltageEa is developed across the tube output impedance IT for application to asucceeding amplifier stage or reproducer.

The amplified radio voltage E which is impressed upon the amplifier tube3 is also impressed upon the control system 0 through the lead 18 andcondenser l9. The tube 4 of the gain control system is not a simplerectifier but comprises a combined pentode amplifier and dioderectifier. The input radio voltage is impressed between control grid G1and the cathode K to produce an amplified radio voltage in the platecircuit, and the amplified voltage is impressed on the anode A throughthe tuned transformer 2B. The cathode circuit of tube 4 preferablyincludes bias resistors 2|, 22 for establishing a delay bias on therectifier AK, the control grid being returned through the leakresistance 23 to the junction of the resistor to place an appropriatebias for amplification on the control grid.

The anode output circuit includes a fixed resistor 24 and an adjustableresistance 25 between the lower terminal of the transformer secondaryand the terminal of resistance 22 which is remote from the cathode. Withthis arrangement, the delay bias on the rectifier is equal to thepotential drop established across series resistances 2|, 22 by spacecurrent flow in tube 4 and, as indicated by the legend +40 volts, theanode A may be so biased that no rectification takes place until thepeak input voltage on the rectifier exceeds 40 volts.

The lead 8 which extends to the controlled tubes of the radio amplifierRF is connected to the anode terminal of resistor 24 through a filtercomprising resistance 26 and condensers 21, and the total direct currentvoltage developed by the control unit C is therefore impressed on one ormore of the amplifier tubes preceding the tube 3. Only that fraction ofthe available direct current voltage which is developed across theresistance 25 is applied through lead 10 to the tube 3 as a gain controlbias.

The close approximation to the flat gain control curve NN of Fig. 2which may be obtained by an appropriate choice of the values ofresistances 24 and 25 is shown by the curve ONN of Fig. 5. The data fromwhich this curve was plotted was obtained when resistor 24 was 500,000ohms and resistance 25 had a value of 130,000 ohms. The control biasapplied to tube 3' was therefore approximately one-fifth of theavailable voltage that was impressed on the controlled tubes ofamplifier RF. Curve ON'B' was plotted from readings made when resistance25 was zero, 1. e., when the Fig. 4 circuit was so adjusted that theautomatic gain control functions in the same manner as the known gaincontrol systems. On the arbitrary scale of values of carrier voltage onthe detector, the desired detector input level had a value ofapproximately 200 and it will be noted that the curve ONB rose toapproximately 600 for a receiver input of /2 volt. As this 3 to 1 riseof detector input amounts to a 9 to 1 rise in the power output at thereproducer, it is apparent that the expected variation in receiver inputproduces substantial changes in the output level when the invention isnot employed. Even greater variations in the output level may obtainwhen the known control systems are not carefully designed to obtain thebest possible gain characteristic.

Curve O'N'N represents the observed relation between receiver input and.detector input iii when the resistance 25 was set at a value of 260,000ohms, i. e., about one-third of the available voltage was impressed onthe control grid of tube 3'. This curve shows the possibility ofobtaining a falling output level with increase in receiver input but, ingeneral, the substantially fiat characteristic ONN' is to be preferred.The particular fraction of the control voltage which should be appliedtoa succeeding tube or tubes 3 will depend upon the type and number ofcontrolled tubes in the amplifier and, for any given receiver design, itis not necessary that the resistance 25 be adjustable since'its value isnot to be varied as an operating adjustment of the receiver.

The problem of receiver design, and particularly in the case of highfidelity receivers, is materially simplified by'the invention as theaudio system is not subjected to relatively wide variations in inputvoltage when resistances 24, 25 are so chosen as to produce asubstantially fiat gain characteristic. The audio amplification may beset at a definite value to obtain the desired output level with theassurance that there will be no danger of overloading the audio system,and thereby introducing distortion, since the top NN' of the controlcurve closely approximates a horizontal line for an extended range ofinput voltages.

The advantage of introducing an independent amplification of the radiovoltage E1 for application to the rectifier is that a sharp control ofthe gain characteristic may be obtained when the voltage E1 iscomparatively low, as is the case when the audio system is designed forhigh gain. When this amplification system includes a circuit tuned tothe frequency of the desired carrier wave, such as the tuned transformer20, there is the further advantage that the gain control system has asharper characteristic of developed voltage versus frequency 011 tunethan does the cascaded amplifier system of the receiver.

The detailed construction and design of the several elements of theradio amplifier and of the control system may be varied at will withoutdeparture from the spirit of my invention as set forth in the followingclaims.

This application is a continuation-in-part of my copending applicationSer. No. 600,750, filed March 23, 1932, for improvements in Automaticgain control.

I claim:

1. In a transmission system, the combination with a plurality of tubesand a circuit network connecting said tubes in cascade for amplificationof radio frequencies, means initially biasing said tubes to establishthe gain thereof in the absence of received signals, and a rectifierhaving an input circuit effectively connected across points of saidnetwork subjected to a radio voltage developed intermediate the inputand output terminals of said amplifier, of means for impressing upon atube preceding and a tube succeeding said intermediate points of saidnetwork direct current bias potentials proportional to the directcurrent potential developed by said rectifier, wherein the initial biason the tube succeeding said network points is less than the valuecorresponding to maximum amplification, whereby the gain of said tubeincreases and thereafter decreases as the magnitude of the directcurrent bias potential impressed on said tube increases towards itsmaximum value.

2. In an electrical wave transmission system, the combination with aplurality of tubes and a circuit network connecting said tubes incascade for transmission of said waves, means initially biasing saidtubes to establish the gain thereof in the absence of waves, a rectifierhaving an input circuit effectively connected across points of saidnetwork subjected to a .wave voltage developed intermediate the inputand output terminals of said system, means for impressing upon a tubepreceding and a tube succeeding said intermediate points of said networkdirect current bias potentials proportionally to the direct currentpotential developed by said rectifier, the initial bias on saidsucceeding tube being less than a value corresponding to maximum gainwhereby the gain of said succeeding tube increases and thereafterdecreases as the magnitude of the direct current bias potentialimpressed thereon increases towards its maximum value.

CHRISTOPHER J. FRANKS.

